Lizards rewind the evolutionary clock but end up the same every time

Different islands but same habitats yielded very similar species.

There’s no doubt that we’ve come a long way in our understanding of evolution since Darwin's time. However, there's still a lot we don’t understand about the processes of natural selection, adaptation, and speciation. One question in particular still looms over the field: how predictable is evolution?

On one side, biologists such as Stephen Gould have long argued that evolution is “utterly unpredictable and quite unrepeatable” due to all sorts of historical accidents and perturbations. "Wind back the tape of life," he wrote, and "the chance becomes vanishingly small" that evolution would proceed in precisely the same way, creating the same set of species. However, in recent years, more and more research has suggested that evolution may actually be quite predictable. New research in the journal Science strengthens this argument by identifying a group of islands where Gould's tape has been wound back multiple times, and evolution has created a near-carbon copy almost every time.

The Greater Antilles are chock full of small lizards called anoles; there are anoles that specialize on twigs, on grasses, and on various parts of trees. Lizards on each island exploit these same niches, and across the entire island chain, anoles that use a particular niche look nearly identical. Grass specialists in Cuba are dead-ringers for those in Jamaica, and anoles that specialize on tree trunks in Puerto Rico could be mistaken for those in Hispaniola or Cuba. The kicker is that each of these islands was colonized separately by just one species of anole. In the forty million years since then, anoles on each island have independently adapted in near-identical ways, right down to the shapes of their tails and the stripes on their sides.

Using various modeling techniques, a group of researchers have shown that this is not a lucky coincidence; instead, evolution may be surprisingly deterministic.

The scientists measured various attributes of 100 lizard species across the Greater Antilles, including measurements of their body size, tail length, limb length, and how many sticky toe-pads they have. After plugging these values into two different models—a Bayesian model and a four-dimensional principal components analysis—the results were clear: species on each island were more similar to those on other islands than would be expected by chance.

But has the whole radiation really repeated itself on each island? To tackle this question, the researchers modeled the “adaptive landscape” of each island, or the various configurations of traits that determine each species’ fitness. They found that there were specific combinations of traits that increased the anoles’ fitness, called “adaptive peaks,” and that anoles on each island tend to converge on these peaks. In these very similar habitats, there are a limited number of configurations that spell success for anoles, producing repeated sets of near-identical lizards.

Over the last 40 million years, there have been ample opportunities for lizards on the four islands to take different evolutionary paths: genetic drift, species invasions, and small climatic differences could have caused divergence. But despite these historical fluctuations, the anoles of the Greater Antilles have evolved in near-parallel ways provided they were occupying the same niche, suggesting that evolution may be more robust to these perturbations than Gould (and others) expected.

(This isn't to say that all the anole species were identical. On the larger islands, Cuba and Hispañola, there are some distinctive habitats that anole species have occupied that aren't shared by the other islands. Those habitats drove distinctive adaptations.)

This work is a remarkable example of convergent evolution on a very grand scale and is a pretty compelling evidence that in at least some cases, evolution can be remarkably deterministic. The Greater Antillean anoles are likely not an isolated case; the authors suggest that the diversification of African cichlids may offer similar evidence of evolution’s predictability. While random genetic mutations may be the fodder for natural selection over the long run, some aspects of evolution may not be very random after all.

Kate Shaw Yoshida
Kate is a science writer for Ars Technica. She recently earned a dual Ph.D. in Zoology and Ecology, Evolutionary Biology and Behavior from Michigan State University, studying the social behavior of wild spotted hyenas. Emailkate.shaw@arstechnica.com//Twitter@KateYoshida

I can see the functional impact of the different ecological niches. To climb a given tree and avoid a given predator a given color pattern, toe spacing, etc. are advantageous.

I'd be curious to learn if these specializations are the result of the same gene expression from species on different islands. The result my be deterministic while the means of achieving that goal may still be truly stochastic.

There would be a distinct possibility that given a similar planetary habitat and planetary evolution, humanoid life could very well have evolved elsewhere in the galaxy/universe.

Therefore, we are "more than likely" not alone.

Sadly, the amount of time a species is viable may prove to be a small fraction of the history of its planet. Therefore, the odds of two nearly similar species coexisting in time and space may prove vanishingly small.

This is a lot like the illusion of choice when selecting skills and talents in an RPG: while the designers intend the players to have hundreds or thousands of possible builds, players will quickly isolate the optimal build and most of them will mimic that one as the best possible choice.

This is a lot like the illusion of choice when selecting skills and talents in an RPG: while the designers intend the players to have hundreds or thousands of possible builds, players will quickly isolate the optimal build and most of them will mimic that one as the best possible choice.

Indeed. The whole point is that these animals evolved in the same or very similar habitats.And then there's surprise that animals adapting to the same habitat adapt in the same way? Is that not how natural selection is supposed to work? Those with the most optimal traits survive.

It seems like specific traits are very much favoured in those habitats, and unsurprisingly the same species adapts to the same habitat in the same way.

This is a lot like the illusion of choice when selecting skills and talents in an RPG: while the designers intend the players to have hundreds or thousands of possible builds, players will quickly isolate the optimal build and most of them will mimic that one as the best possible choice.

Indeed. The whole point is that these animals evolved in the same or very similar habitats.And then there's surprise that animals adapting to the same habitat adapt in the same way? Is that not how natural selection is supposed to work? Those with the most optimal traits survive.

It seems like specific traits are very much favoured in those habitats, and unsurprisingly the same species adapts to the same habitat in the same way.

Furthermore it was the same base species that settled each island. That means each group of genes to tweak their adaptations. It's not like one was a reptile and the other an amphibian that evolved into a reptile and also ended up looking just like the lizards that came from the reptilian ancestor.

The article imply, but does not state outright, that the visually identical lizards on the different islands have different genetics. If not, they could simply have drifted on treetrunks, travelled aboard ships if recent etc and just repopulated. This would mean that the article would be nearly pointeless so I asume that is not the case.

There would be a distinct possibility that given a similar planetary habitat and planetary evolution, humanoid life could very well have evolved elsewhere in the galaxy/universe.

Therefore, we are "more than likely" not alone.

I was thinking the same thing - not only are we not alone, but there is a good chance they look like us.

I don't think that follows at all.

As to whether we're alone, we still don't have any hard evidence on how likely life is to get started in the first place, and how likely it is to lead to an intelligent species in the second. This doesn't significantly change that.

As to whether they look like us, it's one thing to start with a specific gene set and place it in a specific environment and observe convergent evolution. It's something quite different to start the whole process over from the beginning. Consider the vast differences that occurred each time there's been a reset via massive extinction. Dinosaurs and mammals are quite distinct and they shared a common lineage. Consider the differences in mammals on Australia from the rest of the planet. Even if evolution is more predictable than previously believed, I strongly doubt that starting from scratch on a different planet would result in anything resembling us to any significant degree.

There would be a distinct possibility that given a similar planetary habitat and planetary evolution, humanoid life could very well have evolved elsewhere in the galaxy/universe.

Therefore, we are "more than likely" not alone.

I was thinking the same thing - not only are we not alone, but there is a good chance they look like us.

I don't think that follows at all.

As to whether we're alone, we still don't have any hard evidence on how likely life is to get started in the first place, and how likely it is to lead to an intelligent species in the second. This doesn't significantly change that.

As to whether they look like us, it's one thing to start with a specific gene set and place it in a specific environment and observe convergent evolution. It's something quite different to start the whole process over from the beginning. Consider the vast differences that occurred each time there's been a reset via massive extinction. Dinosaurs and mammals are quite distinct and they shared a common lineage. Consider the differences in mammals on Australia from the rest of the planet. Even if evolution is more predictable than previously believed, I strongly doubt that starting from scratch on a different planet would result in anything resembling us to any significant degree.

Even there marsupials evolved into vaguely cat and dog forms. But of course that's still on earth. Planets could have fairly divergent habitats where our adaptations don't make sense.

Isn't it logical that given the same starting point and selection pressures, evolution would produce the same result multiple times? Seems like a not very interesting result to me. Seeing a different outcome each time would be the more surprising result.

There would be a distinct possibility that given a similar planetary habitat and planetary evolution, humanoid life could very well have evolved elsewhere in the galaxy/universe.

Therefore, we are "more than likely" not alone.

I was thinking the same thing - not only are we not alone, but there is a good chance they look like us.

Bipedal sure, there are some advantages to be gained from 2 legoids. 2 arms sure, why waste the calories feeding 4 arms when 2 will do. Looking like us though is betting the long odds even when depending heavily on large numbers. The journey from anole to similar anoles is not very far. The journey from single celled organism to Pauly Shore is not far, the journey from single cell to human has so many branches that it is insane to think they will look like us. Large numbers say that we are not alone.

Isn't it logical that given the same starting point and selection pressures, evolution would produce the same result multiple times? Seems like a not very interesting result to me. Seeing a different outcome each time would be the more surprising result.

There is more than 1 way to skin a cat but they all picked the same way.

I've always thought that when someone takes something that exists or has happened and tries to claim that it's random that the burden of proof is on them. Generally things happen for a reason. So it makes sense to me that evidence is turning up that evolution is more deterministic than it may appear.

Isn't it logical that given the same starting point and selection pressures, evolution would produce the same result multiple times? Seems like a not very interesting result to me. Seeing a different outcome each time would be the more surprising result.

It would seem to me that chaotic processes would also have some influence. So you would think that over time non-critical variations would occur (think of sex selection variations in birds) while variations that affect survival would of course need be similar. As mentioned above though the same starting genome would tend to limit the potential for variation as the "starter kit" would have a certain set of easily changed characteristics built into the genome. It would be very interesting to do genomic studies of the variants to see what percentage of the specific adaptations were accomplished by the same mechanism and which were accomplished by more random kinds of events. I'm thinking of the difference between reshuffle of existing genome versus random genetic mutations.

Isn't it logical that given the same starting point and selection pressures, evolution would produce the same result multiple times? Seems like a not very interesting result to me. Seeing a different outcome each time would be the more surprising result.

It might seem logical to you, but as the article says, it's actually an open question:

the article wrote:

One question in particular still looms over the field: how predictable is evolution? On one side, biologists such as Stephen Gould have long argued that evolution is “utterly unpredictable and quite unrepeatable”

Hence, until someone actually studies it, we don't know. The authors undertook this study not to "surprise", but to address the very question of whether or not we should expect the same result. Their results are important in answering this question.

There would be a distinct possibility that given a similar planetary habitat and planetary evolution, humanoid life could very well have evolved elsewhere in the galaxy/universe.

Therefore, we are "more than likely" not alone.

I was thinking the same thing - not only are we not alone, but there is a good chance they look like us.

Not at all a "good chance." The study considers lizards from 40 million years ago that were placed in very similar environments. However, the initial lizards were still the result of billions of years of change-- with all sorts of random environmental changes.

If another planet very similar to Earth had already evolved primates, then yes, it seems that it would be likely a human-like species would evolve, but this still requires having evolution having yielded such a primate in the first place.

The scientists measured various attributes of 100 lizard species across the Greater Antilles, including measurements of their body size, tail length, limb length, and how many sticky toe-pads they have. After plugging these values into two different models—a Bayesian model and a four-dimensional principal components analysis—the results were clear: species on each island were more similar to those on other islands than would be expected by chance.

"Better than chance" I think would still be expected even in random mutation/evolution because of the presence of environmental pressure. Does the paper expand upon this meaningfully?

I don't think that the expectation of Gould et al would be that you'd end up with lizards with radically different properties when exposed to similar environments. Just that the sequence under which they respond to environmental pressure (and retain the traits that are advantageous) would vary in timing and quantity. You don't end up with identical lizards. You'd naturally select in the same way given the same environment. It isn't clear to me that this is considered in their analysis of "expected/chance," and I'd prefer an expert in the field (ahem, author - your credentials are clearly better than mine here), to perhaps elaborate on how well this is considered in their work.

Evolution itself isn't "told what to do" by the environment - the environment restricts evolution because only useful adaptations result in breeding preference.

Using various modeling techniques, a group of researchers have shown that this is not a lucky coincidence; instead, evolution may be surprisingly deterministic.

This is surprising to ecologists, but probably not to molecular biologists. Adaptation happens most rapidly through simple changes in the regulation/expression of existing genes. More of gene X and your tail gets bigger. Less, its smaller. True novelty often requires the creation of entirely new genes, which is a much more difficult process. You won't see a lizard evolve two tails because that requires a lot more than just changing expression, and that would be much, much harder.

With this in mind its not too surprising that the same traits evolve in response to the same environment given the same initial set of genes. You've got essentially the same pool of readily accessible adaptation. Unless something truly unlikely happens, speciation will tend to explore that same pool of possible choices. If the environment really is the same, the same solution will be optimal. Hence, you'll get very similar animals.

Stephen Gould's quote above reflects a pre-genomic understanding of the basis of evolution. Its not wrong exactly, but it fails to account for the actual machinery of evolution and how it works.

It seems to me that this study could really benefit from mentioning whether the anoles can interbreed and then more extensive genotyping to actually determine how closely related they are. They may very well have done this and I'm planning to read the paper when I'm at work, however if ars left off these details I would be pretty disappointed. Maybe that's why it's a Science paper and not a Nature paper. =p

Isn't it logical that given the same starting point and selection pressures, evolution would produce the same result multiple times? Seems like a not very interesting result to me. Seeing a different outcome each time would be the more surprising result.

It would seem to me that chaotic processes would also have some influence. So you would think that over time non-critical variations would occur (think of sex selection variations in birds) while variations that affect survival would of course need be similar. As mentioned above though the same starting genome would tend to limit the potential for variation as the "starter kit" would have a certain set of easily changed characteristics built into the genome. It would be very interesting to do genomic studies of the variants to see what percentage of the specific adaptations were accomplished by the same mechanism and which were accomplished by more random kinds of events. I'm thinking of the difference between reshuffle of existing genome versus random genetic mutations.

And on that basis, if a certain range of traits are basically easy to evolve into based on the existing genetic configuration, it is possible that evolution will tend to find the same configurations when dealing with the same evolutionary pressures within short periods of time (40 million years counts as short here), while randomness plays a much bigger role dealing with very long periods of time.

That's difficult to prove of course. The islands have an advantage of relatively similar environmental conditions. On a larger scale, you can see examples of convergent evolution e.g. wolves and thylacines https://en.wikipedia.org/wiki/Marsupial_wolf have a lot in common but not as much as these anole species. Are the differences due to increased time, different starting genetics, or different environmental conditions? As in many biology questions, the answer is probably "all of the above" and the question is just about the relative contributions of each.

The article imply, but does not state outright, that the visually identical lizards on the different islands have different genetics. If not, they could simply have drifted on treetrunks, travelled aboard ships if recent etc and just repopulated. This would mean that the article would be nearly pointeless so I asume that is not the case.

In the supplemental material, they construct the phylogenetic trees from mtDNA sequences, so if there was crossbreeding, it would immediately be noticed when pairs of lizard species had near-zero difference between branches.

My guess is that since this analysis is done on existing data sets, someone has already gone through and established which lizards were able to move between islands and which have not. Probably this was done when the initial sequencing was performed. At least that would be most logical.

The reasons modern humans exist on Earth is largely because we changed our diets and developed more complex brains. When early hominids started eating meat in addition to plants, the softer plants allowed us to reduce the size of our jaw muscles. With the smaller jaw muscles, our skulls, and brains, were allowed to increase in size. However, that wasn't all our brains did. It also increased in surface area with the iconic "wrinkles" in the brain. The combination of bipedal movement, easier to digest diets, and increased brain surface area led to our current evolution. Now, if this is an optimal adaptation, given a similar environment, there is a good chance extrasolar intelligent species are of a similar form. They would be bipedal, subsisting on diverse high-calorie diets, and have complex brains with significant surface area.

No, because its still restricted to a finite and probably not all that large set of possible adaptations available in the existing gene pool. And of course its not really deterministic, just constrained over shorter time scales.

Under intelligent design your imagination is the limit since you're not restricted to building on what already exists.

Your review doesn't state that there were no chances that the lizards of one island could not have migrated to another island. Since the sea levels have been dramatically lower during periods of glaciation, this is a critical issue. The genetic differences between island groups would have to demonstrate that there was no common ancestor for the millions of years, and that is extremely doubtful.

What about comparing the saurian fauna of various similar islands without anoles? I mean, you're limiting your sample set to lizards with a relatively recent common ancestor aren't you?

Would you find a lizard substantially like an anole in Australia or in Asia? (The answer is kind of, btw, particularly in Asia). Or would you find analogues for varanids over in South America? (again, kind of. Tegus are similar but have fairly substantial differences).

No, natural selection is performing genetic algorithms, which are also a random heuristic for finding local maximums for complicated optimization problems. Same book, different chapter.

And from that standpoint, the way I'd interpret this is that the fitness function for a given organism in a given environment may have more constrained local maximums, and fewer maximums of similar height, than we may have thought. So Gould could be right that you'll never get the same result twice, but you'll get something pretty dang similar if the constraints are right.

Of course if you start with something completely different then the shape of the fitness function may be completely different -- though it could still be similar in some dimensions, like how there's pretty much always a rabbit-sized animal to fill a rabbit-like niche (even on New Zealand, which lacks native terrestrial mammals and so it's a rabbit-sized flightless parrot that fills the bunny niche). So I wouldn't assume we'll find aliens who look like us except with skin paint and prosthetic forehead bumps.